FIG. 1 shows a conventional bi-level digital printing system 100 having a bi-level digital printing machine 102 and a raster image processor 104 widely used by commercial print shops to print high volume and high quality prints from an input file 80 such as a raster file format (tiff, JPEG, etc.) or a vector file format (pdf, Postscript, etc.). Bi-level digital printing machine 102 has a plurality of fixed print heads (not shown) each having a plurality of fixed nozzles (not shown) that can print a single sized dot or no dot as the paper passes under the nozzles. Raster image processor 104 has an interpretation module 106 and a rendering module 108. As is well known in the art, interpretation module 106 and rendering module 108 have computer instructions or code to produce an unscreened raster image file 110 from input file 80 that is in a non-raster image format (for example, pdf). If input file 80 is a raster file, then input file 80 requires only raster processes, such as scaling and color conversion, to produce unscreened raster image file 110. Examples of conventional bi-level digital printing systems having a digital printing machine and a raster image processor are the HP® Indigo® 20000 digital press sold by Hewlett-Packard Company (www8.hp.com/us/en/commercial-printers/indigo-presses/20000.htm); the KODAK NEXPRESS SX2700 digital press sold by Ricoh Corporation (http://rpp.ricoh-usa.com/products/production-printers/cutsheet/kodak-nexpress); and the Xerox® Color J75 digital press sold by Xerox Corporation www.xerox.com/digital-printing/printers/digital-press/xerox-j75/enus.html).
FIG. 2 shows a generic unscreened raster image file 110 produced by rendering module 108 (w pixels wide by h lines high). Each pixel of unscreened raster image file 110 has a continuous tone value (for example, 0-1023 for 10 bit tone values) for each colorant of Cyan, Magenta, Yellow and black (CMYK). In order to print unscreened raster image file 110 on bi-level digital printing machine 102, raster image processor 104 further comprises a screening module 114 (FIG. 1) that has computer instructions to screen or convert the continuous tone value of each pixel of unscreened raster image file 110 to a print level value (0 or 1) stored in an output bitmap file 116 (FIG. 1) using a well known half-toning algorithm such as threshold screening or AM screening.
FIG. 3 shows a high level flow chart of conventional screening module 114 of raster image processor 104. For each pixel on each line of an input unscreened raster file 110, whose coordinates are X and Y, the continuous tone value is processed into a 1-bit output value by a half-toning algorithm.
Multi level printing machines are being developed where a nozzle can print more than two (2) levels for each printed pixel, such as a four level machine. For example, in a four level machine, each nozzle can print no dot, a small dot, a medium dot or a large dot thereby allowing for a much finer quality print. FIG. 4 shows a generic screened 2-bit output bitmap file for a 4-level digital printing machine. Each pixel has a value of 0-3, with a set of raster data for each printing ink or colorant.
FIG. 5 shows a conventional way of partitioning the tone ranges in equal parts for a 4-level press having 3 dot sizes. One disadvantage of such contiguous sectioning of the printed tone range into equal parts, one for each output tone level for each colorant, is an artifact of flattening or visible loss of screening at the boundaries between the levels.
FIG. 6 is a graph illustrating a response that one would expect for a perfectly built and correctly operating or ideal digital printing machine. For such an ideal press, an input tone value of 33.3% from the unscreened raster file results in a patch of small size dots having a measured print strength of 33.3 percent. However, no digital printing machine operates in an ideal manner.
FIG. 7 is a graph illustrating a response for a digital printing machine that behaves in a non-ideal manner. In this example, an input tone value of 33.3% from the unscreened raster file results in a patch of small size dots having a measured print strength of 45 percent and not 33.3 percent. Further, an input tone value of 66.7 percent results in a patch of all medium size dots having a measured print strength of 55 percent and not 66.7 percent. As such, unwanted artifacts in the print are created as a result of the print heads of the digital printing machine producing a stronger or lighter intensity level for any continuous tone value of the unscreened raster file. These improper values and the intermediate values need to be carefully compensated.
Another drawback with conventional screening modules is the inability to compensate for variations in print strength of individual inkjet nozzles except by modifying the continuous tone input data. Aside from the difficulty in precisely specifying such corrections, for large compensations this can reduce the number of available printed screened tone levels, resulting in undesirable tone banding artifacts. This invention allows for an additional form of compensation that does not reduce the number of printed tone levels, while also simplifying the specification of the individual compensations.